The illumination device is used at night or in situations where raising the level of illumination is desired. It consumes electrical power, and can be switched off in situations where enough illumination is present to save power. In some cases, such as at nightfall or a misty day, illumination is needed but the light from the illuminating device may be stronger than what is needed. In some cases the illumination device could be turned off to save power, however, in other cases the illumination device must be kept on, hence wasting power since the full intensity is generated. If the illumination of the light emitted by the illumination device is adjustable, it could be regulated to the level just required and power could be effectively saved.
For the solid state illumination device, such as light emitting device (LED), the light could be dimmed by several dimming technologies, including phase cut dimming. Compared to other dimming technologies, the phase cut dimming has advantages such as low cost and an agreed standard. Because of the low cost, it is the most popular dimming technology used in the general lighting dimming field. However, with LED lighting, there are problems with the conventional phase cut dimming that needs to be solved.
A conventional phase cut dimming device includes a phase cut for chopping a portion of each waveform of the input voltage signal to change the input power of LEDs, a RC timer for changing the conduction angle of the phase cut, and a trigger for triggering the phase cut. The power factor of the conventional phase cut dimming device decreases rapidly when the conduction angle decreases. For example, the power factor (PF) is lower than 0.25 when the illumination of the LED is adjusted to 25% of the original illumination. On the other hand, while the total harmonic distortion (THD) of the conventional phase cut dimming can be lower than 30%, it is still too large for effective LED dimming.
Low PF and high THD are negatives to the overall performance of LED dimming. Therefore, the PF and THD problems of the conventional phase cut dimming should be solved to raise the effective dimming capability of a phase cut dimming LED driver.